Recently, there has been an increasing interest in energy storage technology. Batteries have been widely used as energy sources in the fields of cellular phones, camcorders, notebook computers, PCs and electric cars, resulting in intensive research and development into them. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, the research and development into a novel electrode and a novel battery that can improve capacity density and specific energy have been made intensively in the field of the secondary batteries.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using an aqueous electrolyte solution (such as Ni-MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, etc). For these reasons, the lithium secondary batteries have been advantageously used. However, such a lithium secondary battery has disadvantages in that organic electrolytes used therein may cause safety-related problems resulting in ignition and explosion of the batteries and that processes for manufacturing such a battery are complicated.
Recently, lithium-ion polymer batteries have been considered as one of the next-generation batteries since the above disadvantages of the lithium ion batteries were solved. However, the lithium-ion polymer batteries have a relatively lower battery capacity than those of the lithium ion batteries and an insufficient discharging capacity in low temperature, and therefore these disadvantages of the lithium-ion polymer batteries remain to be urgently solved.
Such a battery has been produced from many companies, and the battery stability has different phases in the lithium-ion polymer batteries. Accordingly, it is important to evaluate and ensure the stability of the lithium-ion polymer batteries. First of all, it should be considered that errors in operation of the batteries should not cause damage to users. For this purpose, the Safety and Regulation strictly regulate the ignition and the smoking in the batteries.
The lithium secondary battery is short-circuited due to the contact between a cathode and an anode, leading to its explosion with extreme high-temperature heat. The contact between the cathode and the anode may be made, for example, if a porous separator is contracted or melted by the overheat of the battery or penetrated due to the external impacts.
In order to solve the above battery safety-related problem, there has been proposed an electrode having a porous active layer formed by coating at least one surface of an anode or a cathode, and both electrodes with a mixture of inorganic particles and a binder polymer. The conventional porous active layer coated on the electrode shows homogeneous composition morphology toward a thickness direction, as shown in FIG. 2B and FIG. 3B. However, if the electrochemical device is assembled with the electrode having a porous active coating layer, it has disadvantages in that the inorganic particles in the porous active layer are detached during the assembly process such as winding, etc. and a lamination characteristic toward a separator is deteriorated in using a separate porous separator. In order to solve the above disadvantages, if a content of a binder polymer in the porous active layer is increased, characteristics such as the peeling and scratch resistances, the lamination characteristic toward a separator, etc. in the assembly process of the electrochemical device may be rather improved. However, porosities in the porous active layer are decreased since the increase in the content of the binder polymer leads to relative reduction in the content of the inorganic particles, resulting in deteriorated performances of the electrochemical device.